Human Pigmentation and Adaptation
• The body’s largest organ
• Functions in many ways
– Thermoregulation
– Protection from physical and chemical injury
– Protection from invasion by microorganisms
– Manufactures essential nutrient
• As one of the most conspicuous human polytypic variations, skin color has probably attracted more scholarly attention than any other aspect of human variability
• Skin color has served as a primary feature in most systems of racial classification
• Skin color is a polygenic trait, meaning multiple genetic loci are involved in determining skin color
– Multiple genes working together produce a continuous distribution in a “Bell Shape” curve of degrees of light to dark.
– Early models suggested 2 or 4 major genes
• Recent work suggests many genes working together in very complex, additive and non-additive combinations
• The non-enzymatic conversion of dopaquinone into eumelanin and phaeomelanin and their combination into melanosomes is affected by several genetic loci
Human genes have been located by finding homologues to the mouse genes
Sturm, 1998: Table 1
• By the latter half of the nineteenth century, while anthropologists still had no clear idea of the underlying causes of pigmentation, they began to devise measurement techniques to use skin color in racial classification
• Broca established a 34 tone scale, which was simplified by his student Topinard
• These techniques were used into the 20 th century until the introduction of the reflectance spectrophotometer in the early 1950s

• A Reflectance Spectrophotometer shines a light of a specific wave length, using a filter, and measures the intensity of light reflected by the skin
– The technique involves alcohol wash of the skin on the inner upper arm
• allow time for local circulation to return to normal
• shine light and measure reflectance
Skin color is Continuous:
Mean (dot) and s.d. (bar) of skin color, for 22 populations
• The pigments Car otene , Hemoglobin , and
Melanin are involved in skin color
• Carotene, the least common skin pigment results in a yellowing of skin
– Results primarily from the over-consumption of carotene containing foods (like carrots)
– This pigment is significant almost exclusively in pathological or abnormal skin coloration
• Hemoglobin is the complex molecule responsible for transport of oxygen throughout our bodies
– It is the primary protein constituent of Red Blood
Cells
• Oxygenated hemoglobin has a reddish hue
– Produces a pinkish tint to lightly pigmented skin
• Deoxygenated hemoglobin has a purplish color
– Produces the bluish tint to lightly pigmented skin that is characteristic of oxygen deprivation and suffocation
• The primary determinant of variability in human skin color is the amount, density, and distribution of the pigment melanin
• Melanin has a dark brown/purple/black color that is intensified by denser compaction of the melanin granules in the cells of the upper layers of the skin

• The stratum basale consists of columnar cells, the keratinocytes with about 10% of the cells comprising melanocytes
– This is the germinal level of the skin which gives rise to the outer layers of cells and the melanin granules that pigment them
• The stratum spinosum consists of several layers of irregular polyhedral cells
(keratinocytes) flattened on their edges
• The stratum granulosum consists of several layers of flattened polyhedral cells with their long axis parallel to the skin surface
– Cytoplasm contains keratohylin
– As the cells increase in size they die out
• The stratum corneum is composed of a varying number of layers of dead keratinized cells fused to one another except for the outer edge where flaking takes place
Melanocytes synthesize melanin which is combined into granules and injected into the surrounding keratinocytes
Tyr osine
Tyr osinase
Dopa
Tyr osinase
Dopaquinone
5,6-Dihydr oxyindole-
2-car boxylic acid
5,6-Dihydr oxyindole
Dopachr ome
Leucodopachr ome
5-S-Cysteinyldopa
5-S-Cysteinyldopaquinone
Indole-5,
6-quinone
Eumelanin
Benzothiazine
Inter mediar ies
= contributes to copolymerization
Phaeomelanin
• The metabolic pathway to melanin is extremely complicated, involving several intermediate steps
– Starts with the amino acid tyrosine oxidized by the copper-containing enzyme tyrosinase to dihydroxyphenylalanine (dopa) and then to dopaquinone
• A mutation to the gene for the enzyme tyrosinase that produces a protein with decreased functionality will result in a reduced production of melanin
– In the extreme, this produces a genetic form of albinism

– Dopaquinone undergoes a series of nonenzymatic reactions and rearrangements forming the different molecules that are co-polymerized to make up one of the types of melanin
• Eumelanin is the dark brown/purple/black compound found in skin and hair
• Phaeomelanin is the yellow-to-reddish-brown pigment which is present in red hair
– Both forms of melanin combine with other proteins to form the melanosome that is distributed from the melanocyte to surrounding cells
Tyrosinase
Tyrosinase
TRP-1 TRP-2
P Protein
TRP-1
TRP-2
From Sturm, 1998: Figure 1
• The clinal nature of skin color distribution suggests an association with environmental factors varying with latitude
– Ultraviolet Radiation, in particular, the quantity of UV rays striking the surface of the earth from the sun
– Temperature
• Noah’s curse on Ham’s son Canaan
• Response to the heat of the sun
• Bile theory
• Aerobic theory
• Optic theory
• Pigmentation as pathology of adverse environment
• Result of disease
• Vitamin D, Folic Acid, Cold tolerance

• An early explanation of dark skin comes from the biblical story of Noah’s curse (Genesis
Chapter 9, King James Version):
24. And Noah awoke from his wine, and knew what his younger son had done unto him.
25. And he said, Cursed be Canaan; a servant of servants shall he be unto his brethren.
26. And he said, Blessed be the LORD God of Shem; and
Canaan shall be his servant.
27. God shall enlarge Japheth, and he shall dwell in the tents of
Shem; and Canaan shall be his servant.
– Nowhere does this mention darkening of the skin, although other sections of the bible, notably in Job and Isaiah associate curses with darkness
• Lamentations chapters 4 and 5 refer to skin color becoming dark as a result of famine
– Ch 4, v 8: Their visage is blacker than a coal (alternate translation: darker than blackness); they are not known in the streets: their skin cleaveth to their bones; it is withered, it is become like a stick.
– Ch 5, v 10: Our skin was black like an oven because of the terrible famine (alternate translations: terrors or storms)
• By the time the slave trade was actively operating on the West African coast in the mid to late 16 th century, curiosity about the cause of dark skin color was growing
• The accepted environmental explanation was that the action of the sun’s heat was the cause of the differences in the complexion of Europeans and Africans
• Accumulating evidence made it clear that the
“heat of the sun” explanation was not satisfactory
– The skin color of the aborigines of North America in similar climates to Europeans and Africans was neither black nor white, but olive
– Africans were found to vary in color from “black to yellow” according to sources of the day
– Africans forced into European settings were showing no lightening of skin and those Europeans living in
Africa were not appreciably darkening
• The view emerged that the African’s blackness was innate and permanent
• This English physician published his view in his
1646 Pseudodoxia epidemica :
– “If the fervour of the Sun, or intemperate heat of clime did solely occasion this complexion, surely a migration or change thereof might cause a sensible, if not a total mutation; which notwithstanding experience will not admit. {Despite their transplantation, there remains among their descendants} a strong shadow of their
Originals: and if they preserve their copulations entire, they still maintain their complexion. . . .{L}ikewise, fair or white people translated in hotter Countries receive not impressions amounting to this complexion”
– He also dismissed the Ham’s curse explanation as a foolish tale
• Attempts made to resolve whether differences in skin color was due to innate differences in biological structure or transient response to the sun’s heat
– Santorio Santorio (1561-1636): 1614 De statica medicina skin’s complexion was determined by the presence of black bile
• Revival of a notion from the ninth century Arabian physician
‘Ali al-Tabari in his medical compendium Paradise of
Wisdom
– Supported by Italian anatomist Malpighi (1687)
• Determined that dermis and stratum corneum is colorless in both Balcks and Whites, he agreed with Santorius that the blackness of Africans must originate in the underlying mucous and reticular body—colored by bile
• Early Eighteenth century anatomists reported the presence of black bile in the Malpighian layer

• Scholars attempting to avoid the polygenistic implications of the bile theory clung to the work of the French surgeon Littré who failed to find a black gelatinous bile in the malpighian layer of
Africans
• Monogenists began advocating a more complex view of environmental causes that harks back to
Hippocrates’ Airs, Waters, Places focusing on the differential qualities of the atmosphere
– This “aerobic” theory was championed by Du Bos in
1719 and picked up by others, including Blumenbach in his Varieties of Man in 1795
• Le Cat 1765 discerned black deposits he called
“æthiops”in nerve tissues of animals
– Without having any idea about the origin or role of these æthiops he was convinced that there was a relationship with the environment and he conjectured that Negroes had more of the structures than Europeans did, making them the basis of the dark complexion of the Africans
• Patot 1733 claimed that complexion differences were based on the ability of the human skin to transmit light—the “optic” theory of skin color
• John Mitchell (1744) published the first major scientific study of skin color in the New World
– Compared skin of colonial Whites and African slaves in
Virginia
• Concluded there was no anatomical basis for the bile theory
• Only structural difference was thicker skin among the Africans
– Applied Newtonian optics to suggest that skin color was based on thickness of the skin and its ability to transmit light
– As thickness increased, the skin appeared darker
• African’s thick skin prohibited the transmission of any color through them
• He also conjectured that the original skin color of man was neither black nor white but something in between with
Europeans and Africans represented divergent extremes caused by the degenerative influences of the environment
• Buffon argued that skin color differences were reflective of the geographical degeneration of the primordial type
– He suggests restoration of the “degenerate races to the purity and vigor of the original type” would require the transplantation of the these people to a more temperate zone plus a change of diet and a long span of time
• Lafitau (1724) argues that dark skin color is a congenital malformation—already present in
African fetuses
• Rush (1795) claimed the skin color of the Negro was derived from leprosy
– He argues that Africans suffer from a congenital disease so mild that excess pigment was the only symptom
• Albinism and other depigmentation conditions among Africans were seen as reversion to the original complexion
• Samuel Stanhope Smith (1810) used the depigmentation of Henry Moss, an African
American from Virginia to suggest that pigmentation was nothing more than an “universal freckle”, occasioned by environmental exposure to sunlight
• He argued that climate affected skin color
– Evidence: the darkening and lightening of complexion with the seasons
– Cold air “chafes the countenance and increases the ruddiness of the complexion”

• Late in the 19 th century the process of melanization in plants was discovered to be dependent on the enzyme tyrosinase
• German histochemist Bruno Bloch demonstrated in 1927 that this same pathway was involved in pigmentation of human skin in vitro
• By 1950 the cells where these reactions took place, the melanocytes, were identified and it was found that these cells originate embryonically in the neural crest
• By 1955 the UV protective effects of melanin in the skin were demonstrated
th
• The discovery of the interaction between melanin and UV led to a number of explanations of skin color
• The association between vitamin D synthesis, UV radiation, and skin pigmentation was suggested in the 1930s but it was largely ignored until revived by Loomis in 1967
– Loomis proposed that the rate of vitamin D synthesis is governed by pigmentation and keratinization which affect the amount of UV penetrating to the stratum granulosum , making skin color responsive to UV levels
• Selection favoring high levels of melanin pigmentation in areas of high Ultraviolet
(UV) radiation may involve several selective agents
– Sunburn can cause skin lesions and infections, preventing some degree of heat loss
– Also predisposes to skin cancer
• Highly pigmented skin provides partial protection so selection would favor dark skins in high UV areas
– Skin cancer is found to be prevalent among light skinned individuals in tropical latitudes
– In Nigeria and Tanzania no albino over the age of 20 years was found to be free of malignant or pre-malignant skin lesions
– In Tanzania chronic skin damage was found in every albino infant by the end of the first year of life
• This high rate and early evidence of skin damage suggests that cancer may have been a strong selective pressure in tropical areas
– UV light causes denaturization (a chemical breakdown) of Folic Acid circulating in the blood
• This can induce a deficiency even if the diet supplies adequate folic acid
– Deficiency symptoms include anemia, infertility, and birth defects, especially neural tube defects
• High melanin content in the epidermis can protect circulating Folic Acid, thereby selecting for dark skin in low latitude areas
– Ultraviolet light is known to suppress immune function
• It has been hypothesized that increased melanization could protect the immune system by shielding the blood borne components of the system from UV
• A recent study demonstrated that differences in skin color were not associated with differences in UVinduced immune deficits

• Selection favoring low levels of melanin pigmentation in ecosystems where there are low levels of UV radiation (primarily in the high latitudes) may also involve different factors:
– The regulation of Vitamin D synthesis
– Frost bite sensitivity and cold tolerance
– Regulation of Vitamin D synthesis
• Vitamin D in the body is derived primarily from the skin and secondarily from the diet
• Vitamin D is synthesized in skin by the action of
UV-B
– Precursor molecule is 7-DHC or 7-dehydrocholesterol, which occurs in the strata granulosum and basale of the epidermis
– UV-B exposure causes a photochemical (non-enzymatic) conversion of 7-DHC into previtamin D
• High melanin content in skin reduces UV-B exposure and cuts photochemical conversion
– Previtamin D is transformed into vitamin D by a temperature dependent process over 2-3 days
– Vitamin D then diffuses into the blood vessels of the dermis
– The liver and kidney further transform the Vitamin D into
1,25-dihydroxyvitamin D which is the most active form of the vitamin
• The function of Vitamin D is to actively cause calcium absorption across the wall of the small intestine into the blood stream
• Calcium is used for bone and tooth development as well as for nervous and muscle action
– The skeleton serves as a calcium reservoir
– If calcium levels in intracellular fluid drops, hormones are released to cause resorption of bone, placing calcium into circulation
• A deficiency of Vitamin D in infants and children causes
Rickets, in adults a deficiency causes osteomalacia
– Rickets refers to a defect in the calcification of growing bone so that the bones are structurally weak and unable to withstand mechanical pressure
• Symptoms include muscle weakness, deformity of the long bones including bowed legs, knuckle-like projections along the rib cage
(rachitic rosary), deformities of the pelvis that are often permanent
– Long bone deformity impairs locomotion
– Pelvic distortion can make childbearing dangerous--potentially killing mother and baby
• Prior to widespread Vitamin D supplementation in the 1930s, Black women in the U.S. showed nearly 8 times greater pelvic deformity than
White women

• Assuming that our ancestors had dark skin in the tropics, as hominids moved into higher latitudes there would have been substantial selection favoring lower melanin content in the skin to improve Vitamin D synthesis
• Counter arguments:
– Robins (1991) maintains that there is no evidence of rickets in northern zones of North America, where skin color was presumably dark—including among the Eskimo
• Also argues that substantial storage of Vitamin D would make skin lightening in response to lower UV unlikely
– Need to consider the effects of clothing on northern latitude populations, where selection for dark skin would be lessened and the diet may contribute more Vitamin D
– Frost bite sensitivity and cold tolerance
• As our ancestors moved into higher latitudes they also would have been subject to colder temperatures
• There is a great deal of anecdotal evidence and some medical record evidence (most from the Korean
War) suggesting that individuals with heavily pigmented skin are more susceptible to frostbite
• Animal studies demonstrate that melanocytes are more easily destroyed by freezing than the rest of the skin cells
• Experiments with guinea pigs, cooling both dark and light skinned areas on the same animal, showed dark skin more susceptible to frost bite
• Frost bite cripples hands and feet causing survival problems, and secondary infections including gangrene may be fatal
• Tanning is a two-stage acclimatizational response of the skin to increasing levels of
UV exposure
– Immediate tanning is the transient brownish tan following exposure to UV-A and visible light
• Reaches a maximum within 1-2 hours after exposure
– Fades between 3-24 hours after exposure
• No new melanosomes formed, so the likely mechanism is the photo-oxidation of existing melanin or other epidermal elements
– Delayed tanning is the durable browning caused by repeated exposure, primarily to UV-
B but UV-A and visible light also play a role
• Gradual process of skin darkening starting 48-72 hours after irradiation
– Reaches a maximum 19 days after an exposure
– Requires 9½ months for skin to return to original melanin content
• Melanocytes enlarge, increase dendrite density, and experience other internal changes
• Melanosomes increase in number and melanization
• Blangero J, and Williams-Blangero S (1992) Quantitative genetic analysis of skin reflectance: a multivariate approach. Human Biology, 64:35-49
• Jeevan A, and Kripke ML (1993) Ozone depletion and the immune system. The Lancet, 342:1159-1160
• Montellano, BR Ortiz (1993) Melanin, Afrocentricity, and pseudoscience. Yearbook of Physical Anthropology, 36:33-
58
• Robins AH (1991) Biological perspectives on human pigmentation. Cambridge: Cambridge University Press
• Sturm, RA (1998) Human pigmentation genes and their response to solar UV radiation. Mutat. Res. Rev., 422:69-76
• Spencer, F (1997) Skin color. In: F Spencer (ed.), History of
Physical Anthropology, Vol. 2. Pp. 945-955.